mmc: core: Fix use of uninitialized data in mmc_cmd_app.

mmc_cmd_app did not zero out mmc_command on stack.

Reported-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Andrei Warkentin <andreiw@motorola.com>
Signed-off-by: Chris Ball <cjb@laptop.org>